An awesome article about maintaining availability of microsoft.com.. check it out :
There is, I think, an insane lead-in
According to the Keynote reports, Microsoft.com has been available more than 99.8 percent of the time for the past five consecutive years, and more than 99.9 percent of the time for the past two years. The site generates more than 1.2 billion hits per day from more than 57 million unique Internet Protocol (IP) addresses. This traffic generates 200 million daily page views, averages 30,000 Hypertext Transfer Protocol (HTTP) requests per second, and results in an average of 750,000 concurrent client connections.